1. Field of the Invention
The present invention is directed to a fuel cell system, and in particular to a fuel cell system to be mounted on an automotive vehicle.
2. Description of the Related Art
In general, in fuel cell systems, the consumption ratio of hydrogen contained in a fuel th gas which is supplied to a fuel-cell stack is not 100% but about 80%. Thus an off gas of the fuel gas contains a considerable amount of hydrogen. In the pending United States patent application, filed Jun. 1, 1999 and having Ser. No. 09/323,551 now U.S. Pat. No. 6,306,532, such an off gas is burned in a combustion device and an energy of the resultant exhaust gas is used to drive a turbine. The turbine is connected to a compressor which supplies an oxidizing agent gas to a fuel-cell stack. Thus, the electric power required for driving an electric motor which turns the turbine can be reduced.
In the foregoing fuel cell system, the supply of the oxidizing agent gas to the reforming device is established by another turbine driven by an electric motor. Thus there is a need to reduce the electric power consumed by this electric motor as well.
It is, therefore, a primary object of the present invention to provide a fuel cell system which meets such a need.
In order to attain the foregoing and other objects, a first aspect of the present invention provides a fuel cell system which comprises a fuel-cell stack which generates electric power by utilizing a fuel gas and an oxidizing agent gas; a combustion device which bums an off gas of the fuel gas emitted from the fuel-cell stack; an oxidizing agent gas supply device including a turbine and a compressor, the turbine being rotated by combustion energy of a gas exhausted from the combustion device, the compressor being coupled to the turbine to pressurize the oxidizing agent gas, the oxidizing agent gas supply device being connected to the fuel-cell stack for supplying the pressurized oxidizing agent gas; a reforming device which reforms a carbon hydride family fuel into the fuel gas; and oxidizing agent gas pressurizing means for further pressurizing the pressurized oxidizing agent gas, the oxidizing agent gas pressurizing means being connected between the reforming device and the fuel-cell stack.
A second aspect of the present invention provides a modification of a fuel cell system according to the first aspect of the present invention in such a manner that the reforming device includes a reforming portion which reforms the carbon hydride family fuel into the fuel gas and a CO-reducing device which reduces a CO component in the fuel gas, the reforming portion and the CO-reducing portion of the reforming device being connected to the oxidizing agent gas pressurizing means by way of flow rate control means.
A third aspect of the present invention provides a modification of a fuel cell system according to the second aspect of the present invention in such a manner that the flow rate control means is in the form of a three-way valve.
A fourth aspect of the present invention provides a modification of a fuel cell system according to the second aspect of the present invention in such a manner the flow rate control means is in the form of two flow rate control vales.
A fifth aspect of the present invention provides a modification of a fuel cell system according to the first aspect of the present invention in such a manner that the oxidizing agent gas pressurizing means is in the form of two oxidizing agent gas pressurizing devices, the reforming device includes a reforming portion which reforms the carbon hydride family fuel into the fuel gas and a CO-reducing device which reduces CO component in the fuel gas, the reforming portion and the CO-reducing portion of the reforming device being connected to the oxidizing agent gas pressurizing devices, respectively.
A sixth aspect of the present invention provides a modification of a fuel cell system according to the first aspect of the present invention in such a manner that the fuel cell system further comprises a gauge which determines a pressure difference between oxidizing agent gas and the fuel gas which are to be supplied to the fuel-cell stack.
In accordance with the first aspect of the present invention, since the oxidizing agent gas to be supplied to the reforming device from the oxidizing gas supply device has been further pressurized, a balanced pressure condition can be established between the fuel gas and the oxidizing agent gas which are to be supplied to the fuel-cell stack, which improves the durability of the solid state polymer electrolyte membrane and prevents supplying the oxidizing agent gas to the combusting device, with the result that the supply of the oxidizing agent gas to both the fuel-cell stack and the reforming device from the oxidizing agent gas supply device becomes possible, thereby reducing the electric power consumed in the fuel cell system.
In accordance with the second aspect of the present invention, the flow rate of the oxidizing agent gas supplied to the reforming portion of the reforming device and the flow rate of the oxidizing agent gas supplied to the CO-reducing portion of the reforming device can be adjusted or varied independently, which makes it possible to separately adjust the flow rate of the oxidizing agent gas supplied to each of the reforming portion and the CO-reducing portion of the reforming device.
In accordance with the third aspect of the present invention, a three-way valve is employed as the flow control means. The three-way valve is commercially available and is simple in construction, which makes it possible to independently control the flow rate in an easy way.
In accordance with the fourth aspect of the present invention, two flow rate control valves are employed as the flow control means. Such valves are commercially available and are simple in construction, which makes it possible to easily provide independent flow rate control.
In accordance with the fifth aspect of the present invention, the flow rate of the oxidizing agent gas supplied to the reforming portion of the reforming device and the flow rate of the oxidizing agent gas supplied to the CO-reducing portion of the reforming device can be adjusted or varied independently by the two oxidizing agent gas pressuring devices, which makes it possible to independently adjust the flow rate of the oxidizing agent gas supplied to each of the reforming portion and the CO-reducing portion of the reforming device.
In accordance with the sixth aspect of the present invention, the pressure differential gauge determines the pressure difference between the oxidizing agent gas and the fuel gas, which makes it possible to establish suitable control of each of the oxidizing agent gas supply device, the oxidizing agent gas pressurizing device and the flow rate control means.
According to the present invention, problems can be solved which occur when the oxidizing agent gas is fed from the common oxidizing agent gas supply device to both the reforming device and the fuel-cell stack. Due to the fact that the pressure of the fuel gas generated at the reforming device depends on the pressure of the oxidizing agent gas which is supplied to the reforming device, upon employment of the foregoing common oxidizing agent gas supply device, a pressure difference exists between the oxidizing agent gas supplied directly from the oxidizing agent gas supply device to the fuel-cell stack and the oxidizing agent gas supplied indirectly via the reforming device from the oxidizing agent gas supply device to the fuel-cell stack.
A pressure difference also results from the pressure loss of the oxidizing agent gas in the reforming device. If the foregoing pressure difference is too large, the solid-state polymer electrolyte membrane is applied with excess stress, which causes a serious durability problem. If the oxidizing agent gas supply device is driven by the turbine, the off gases of both the oxidizing agent gas and the fuel gas are combusted in the combusting device. When the oxidizing agent gas is supplied from the oxidizing agent gas supply device to the fuel-cell stack while leaving its pressure unchanged and the off gas of the oxidizing agent gas is supplied to the combusting device, the resultant burning or combustion pressure becomes high, the amount of power or energy recovered by the turbine is increased.
However, as previously explained, due to the pressure loss at the reforming device, the pressure of the off gas of the fuel gas becomes correspondingly low when supplied to the fuel-cell stack, so that the off gas of the fuel gas will not reach the combusting device in which the pressure of the oxidizing agent gas is higher than the pressure of the off gas of the fuel gas. Furthermore, the oxidizing agent gas can move toward the fuel electrode side of the fuel-cell stack by reverse flow through the conduit which is used to the supply of the off gas of the fuel gas to the fuel-cell stack.
In view of the foregoing circumstances, for establishment of a balanced pressure condition which allows the off gas of the fuel gas to be supplied into the combusting device, it is necessary to equalize the pressure of the oxidizing agent gas and the pressure of the fuel gas which are to be supplied to the combusting device by providing a throttle valve in the conduit through which the oxidizing agent gas moves to the combusting device. However, this increases the flow resistance in the conduit, which decreases the discharged amount of the oxidizing agent gas from the oxidizing agent gas supply device.
In order to solve such a problem, the invention further pressurizes the fuel gas by increasing the pressure of the oxidizing agent gas inputted into the reforming device. Thus, the oxidizing agent gas to be supplied to the reforming device is obtained by using the conduit which branches from the oxidizing agent gas supply device for the fuel-cell stack and the resultant conduit is provided therein with an oxidizing agent gas pressurizing device which increases the oxidizing agent gas pressure by a desired amount before being supplied to the reforming device.
Thus, the pressure balance in the gas supply system is solved and the flow rate control or adjustment of the oxidizing agent gas which is to flow into the reforming device becomes easier. In addition, the oxidizing agent gas pressurizing device does not need to pressurize oxidizing agent gas or air at atmospheric pressure but further pressurizes the already pressurized oxidizing agent by approximately 0.5 kgf/cm2, which does not require as much electric power.